The present invention relates to a signal converter and a process control signal output circuit in which signals received from various types of sensors are converted into electric signals for easy handling thereof, and in particular, to a signal converter for the process signal measurement especially in a case in which a signal converter suitable for a multi-point input operation of a temperature converter employing a thermoresistance and a thermocouple is to be universalized for a multi-range operation and in which process control signal output modules are installed at a large number of points for the process control operation.
In procedures involving process control operation, various kinds of sensors such as transmitters and converters to measure pressure and/or differential pressure and thermocouples and thermoresistances to sense temperature are installed in a plant such that measured values from the sensors are received by a host computer to monitor the state of the plant to thereby control the operation of the plant in accordance with the measured values. The values sent from the sensors cannot be directly processed by the host computer. Signals representing the measured values from the sensor are required to be transformed into, for example, specified signals ranging from one (1) dc volt to 5 dc volts. A signal converted is ordinarily disposed between the sensors and the host computer for the signal matching operation therebetween.
Additionally, although the converter handles input signals from the sensors to the host computer, when the host computer transmits to terminals, e.g., valves control signals resultant from process control operations for values of proportion, integration, and differentiation (PID), namely, when the computer processes control output signals ranging from 4 dc milliampere (mA) to 20 dc mA or from 1 dc V to 5 dc V, there is usually installed a multi-point control output unit in addition to the signal converter in the plant.
Description will be given of a conventional example of system constitution by referring to a simple plant configuration shown in FIG. 5. The example includes two loops each accomplishing a simple process to control operation of a boiler in which fuel is fed to the boiler to regulate its steam temperature.
FIG. 5 includes a host computer 201 to conduct control arithmetic operations such as PID calculations, a process input/output (PIO) unit 502 which conducts an analog-to-digital (A/D) conversion to transform analog signals from a converter unit into digital signals to thereby serve as a communication interface for the host computer 201, an analog input board 503, an analog output board 504, a communication interface 505, a power supply 506, and a communication cable 507. Moreover, there are included a signal converter unit 508 to convert signals from sensors, signal converter modules 509 to 512, an interface 513 to receive analog signals from plural converter modules to connect the signals to the input board 503, a power supply 514, and a signal cable 515. FIG. 5 further includes a terminal strip unit 516 to couple an output signal from the output board 504 with a processing unit, terminal strips 517 and 518, and interface 519 for signal transmission. The unit 516 is linked with a plurality of terminal strips for, ordinarily, 8, 16, or 32 points. The strip includes an external connection terminal which connects a control valve or the like and which conforms to M4 screw specifications in ordinary cases. The terminal is independently disposed, not mounted on the PIO unit 502. The system further includes a flow (rate) meter 221, a control valve 222, a temperature sensor terminal 223, and a boiler 224. Operation of the configuration will now be described.
First, signals from the flow meters 221-1 221-2 and the temperature sensor terminal ends 223-1 and 223-2 are fed respectively to the converter modules 509 to 512 of the unit 508 for conversion thereof. The unit 508 is linked with a plurality of terminal strips for 8, 16, or 32 points. Signals from the respective modules are fed to the interface 513 to be supplied via the cable 515 to the input board 503 of the unit 502. The input board 503 converts an analog input signal from the converter unit 508 into a digital value. The process signal representing the digital value is transmitted via the interface 505 to the host computer 201.
Receiving the process signal, the computer 201 executes an arithmetic operation such as the PID operation to thereby attain a control output value. The value is then inputted via the cable 507 and the interface 505 to the analog output board 504. The board 504 transforms a plurality of digital values into analog signals to produce control output values corresponding to outputs of first-loop and second-loop operations. These output values are supplied via the cable 520 and the interface 519 to the terminal board unit 516 to be fed therefrom via the terminal boards 517 and 518 to the control valves 222-1 and 222-2, respectively.
Each process of the first and second loops is a simple example in which fuel is supplied to the boiler to control the steam temperature thereof. As above, there is constructed a control loop in which the steam temperature and the flow rate of fuel are measured and the PID operation is conducted for the measured values to supply control output signals to the valves.
Next, description will be given in detail of the converter modules 509 to 512 of the unit 508 in the system.
Various types of sensors are connected to the sensing terminal points and obtained signals vary within various ranges. In the converter module, consequently, the gain and bias values of an amplifier circuit thereof are required to be set and adjusted for each sensor. If electric insulation is required, it is necessary to provide an insulating circuit.
Description will now be given of the conventional converter modules utilizing a thermocouple as its sensor (specifically, a K-type thermocouple with an operating temperature ranging from 300.degree. C. to 600.degree. C.).
The first converter module will now be described. FIG. 3 shows constitution of the module.
FIG. 3 includes an input terminal 1, an initial-stage amplifier 2, a gain setting resistor 3 to set the gain of the amplifier 2, a bias power supply 4, a bias setting circuit 5, an insulating circuit 6, an output circuit 7, and an output terminal 8.
First, the thermocouple signals corresponding to temperature values ranging from 300.degree. C. to 600.degree. C. are transformed into voltage signals ranging from 1 dc V to 5 dc V to be inputted to the PIO unit 502. In the conversion, values of thermoelectromotive force of the thermocouple ranging from 12.207 mV to 24.902 mV are multiplied by about 315 to obtain voltages ranging from 3.846 V to 7.846 V. Adding thereto a bias value of -2.846 V, there are obtained voltage values ranging from 1 dc V to 5 dc V. Consequently, when the K-type thermocouple with the operating temperature ranging from 300.degree. C. to 600.degree. C. is adopted as the sensor, it is required to set the default values beforehand, i.e., 315 as the gain setting value and -2.846 V as the bias value. The first converter module is therefore initialized as follows. The gain setting resistor 3 is first appropriately adjusted, the gain value of the amplifier 2 is set to 315, and then the bias power supply 4 and the bias setting circuit 5 are adjusted to set the bias value to -2.846 V.
As above, in the configuration example of the first converter module, the gain and bias values are calculated beforehand in accordance with the type of the sensor and the range of input signal values to thereby set and adjust the circuit constants.
Referring next to FIG. 4, description will be given of a configuration example of the second converter module including a microcomputer.
In FIG. 4, the same components as those of FIG. 3 are designated by the same reference numerals. The configuration includes an input terminal 1, an initial-stage amplifier 2, an output circuit 7, an output terminal 8, an analog-to-digital (A/D) converter 9, a digital signal processing circuit 10 including a microcomputer, an insulating circuit 11, and a digital-to-analog (D/A) converter 12.
In this example, the sensor type and the signal range can be set by the processing circuit 10. While the gain setting resistor and the bias power supply are set to select only the necessary signal range for each sensor type in the first converter module, the measuring ranges of particular sensors such as thermocouples and thermoresistances are set to their full-span values to select only the necessary signal ranges through arithmetic operations by the circuit 10. For example, in the measuring ranges of the thermocouples, the thermoelectromotive force takes values of from -10 mV to 80 mV. In accordance with the input values in this range, an signals which can be inputted to the second converter module. For example, when it is assumed that the input signal is multiplied by 89 in the amplifier 2 and a bias voltage of 1.9 V is added to the amplified value, signals ranging from -10 mV to 80 mV are converted into signals ranging from 1 V to 9 V. Assume that the A/D converter has an input range of from 0 V to 10 V and that a range of from 0 V to 1 V and a range of from 9 V to 10 V constitute an underflow zone and an overflow zone, respectively. With this provision, the module can cope with any kinds of thermocouples including K-type and E-type thermocouples such that the other necessary setting operations are achieved through arithmetic operations.
The processing circuit 10 includes an area to store therein the sensor types and signal ranges; moreover, there are disposed data tables for linearization for a plurality of sensors. As correction data, for example, for thermocouples, values of thermoelectrodynamic force are defined in the Japanese Industrial Standard (JIS). When these values are set beforehand to a data table of correction data, interpolation can be easily conducted in the linearization with the data.
In this configuration, as in the first converter module, when a K-type thermocouple with an operation range of from 300.degree. C. to 600.degree. C. is assumed to be connected to the input terminal, the thermocouple type and the signal range are respectively set in advance to "K type" and "from 300.degree. C. to 600.degree. C." in the processing circuit 10. It is defined in the circuit 10 that the input zero point is set to 12.207 mV and an output of 1 dc V corresponds to 300.degree. C.; moreover, the input span point is at 24.902 mV and an output of 5 dc V corresponds to 600.degree. C. The control operation with respect to ranges and the output processing are achieved under this condition. In the data table, a portion thereof related to the range of from 300.degree. C. to 600.degree. C. is selected for the correction.
The second converter module constructed as above can produce desired output signals only by inputting thereto sensor types and signal ranges. Namely, the module is not required to calculate the circuit constants to set the constants therein in accordance with the sensor types and signal ranges.
The first and second converter modules described as examples of the prior art have the following aspects.
Each time a sensor type and a signal range are altered in the first converter module, the gain and bias values are required to be calculated for the setting and adjusting of the circuit constants.
However, a large number of converter modules are employed in the field of process signal measurement. It is therefore a common practice to adopt a block-type converter module like the converter unit 508 of FIG. 5 in which converter modules are classified into groups for 8, 16, or 32 points and which is advantageous in reduction of the installation space and the wiring cost. As a basic element of the multi-point signal converter unit in the configuration above, although the first converter module requires for each point the setting and the adjusting of the gain and bias values at the circuit level, the overall circuit can be constructed at a relatively low cost.
Unlike the first converter module, the setting and the adjusting of the gain and bias values need not be conducted at the circuit level for each alteration of the sensor type and signal range in the second converter modules. Using a high-precision A/D converter and a microcomputer, there can be constructed a converter module which can be appropriately operated only by inputting the sensor types and signal ranges. However, an A/D converter, a microcomputer, and a D/A converter are necessary for each point. When used in a multi-point signal converter facility of the above structure, the second converter module increases the overall cost of the system.
In each of the groups of the first and second converter modules, even when the design values of gain and bias are the same therein, an error of several percent generally takes place due to fluctuation in quality of parts of the respective modules. Conventionally, to correct the error, a variable resistor or the like is arranged for the pertinent module, which has been disadvantageously troublesome.